A laser is a device which has the ability to produce monochromatic, coherent light through the stimulated emission of photons from atoms, molecules or ions of an active medium which have typically been excited from a ground state to a higher energy level by an input of energy. Such a device contains an optical cavity or resonator which is defined by highly reflecting surfaces which form a closed round trip path for light, and the active medium is contained within the optical cavity.
If a population inversion is created by excitation of the active medium, the spontaneous emission of a photon from an excited atom, molecule or ion undergoing transition to a lower energy state can stimulate the emission of photons of substantially identical energy from other excited atoms, molecules or ions. As a consequence, the initial photon creates a cascade of photons between the reflecting surfaces of the optical cavity which are of substantially identical energy and exactly in phase. A portion of this cascade of photons is then discharged out of the optical cavity, for example, by transmission through one or more of the reflecting surfaces of the cavity. These discharged photons constitute the laser output.
Excitation of the active medium of a laser can be accomplished by a variety of methods. However, the most common methods are optical pumping, use of an electrical discharge, and passage of an electric current through the p-n junction of a semiconductor laser. Semiconductor lasers contain a p-n junction which forms a diode, and this junction functions as the active medium of the laser. Such devices are also referred to as laser diodes. The efficiency of such lasers in converting electrical power to output radiation is relatively high, and for example, can be in excess of 40 percent.
In order to effect optical pumping, the photons delivered to the lasant material from a radiant source must be of a very precise character. In particular, the pumping radiation must be of a wavelength which is absorbed by the lasant material to produce the required population inversion.
The flow of current through a laser diode perturbs the electron population in the valence and conduction bands. The energy gap between the lowest empty level in the valence band and the lowest filled level in the conduction band is altered. The net effect is that the output wavelength is dependent on the driving current. The wavelength increases with increasing drive current. For gallium aluminum arsenide laser diodes, the rate of increase is typically 0.025 nm/mA.
The output wavelength is highly dependent on the detailed electronic distribution of the valence and conduction bands. Consequently, output wavelength is a function of the temperature of the junction. The emitted wavelength increases if the temperature of the junction is increased. Typically the emission wavelength changes by 0.3 to 0.4 nanometers per degree centigrade. Clearly, if a stable output wavelength is required, the temperature of the laser diode must be maintained at a constant level. This is usually achieved by using a small thermoelectric cooler unit, a thermocouple sensor and a feedback circuit.
The gain of any lasing medium is a function of the population inversion ratio. This is actually a ratio of the perturbed population distribution to the equilibrium (Boltzmann) distribution. As the temperature of a laser diode junction rises, the natural Boltzman population distribution of the electrons changes and even more electrons are required in the conduction band to achieve the same effective population inversion. Therefore, for a fixed driving current, increasing the temperature of the laser diode will normally decrease its output power.
Laser diode lifetimes in excess of 50,000 hours are not uncommon. However, there are certain factors which can have a drastic effect on this. Both high device temperature and sudden current spikes can be fatal to laser diodes.
Device failure can be either sudden and catastrophic, or a gradual degradation of performance. The gradual degradation process can be due to the accumulation of crystalline flaws in the active junction region. These can be small or large, but all have their origin as missing atoms or extra (interstitial) atoms in the lattice. At these so-called lattice defects, there is a discontinuity in the band structure which can allow electrons to "leak" from the conduction band down to the valence band without emission of a photon. The excess energy is instead released non-radiatively as vibrational energy of the lattice. Continual driving of a laser diode near its damage threshold, sudden spikes in the driving current, and failure to maintain a reasonable junction temperature, can all lead to an increase in the number and size of the lattice defects in the junction.
The temperature of a laser diode rises above ambient temperature during normal operation for two reasons. Firstly, the semiconductor is heated by simple resistive heating. Secondly, the internal photon flux may be reabsorbed, particularly by impurities. Clearly, to prolong the life of a laser diode it is advantageous to cool the diode in some way.
Device failure can also result from degradation of the output facet. This can be sudden or gradual. It is caused by thermal effects, sometimes in conjunction with thermal oxidation. Large spikes in the driving current can produce bursts of heat which exceed the heat dissipation capacity of the device. This may cause fatal damage or fractures to the output facet.
It is therefore very important to control the temperature of diode lasers since: (1) a diode laser generates an enormous amount of waste heat per unit volume and temperature significantly affects, alters and changes the characteristics of laser diodes by changing the wavelength of the output radiation of laser diode pumps; (2) the lifetime of a laser diode is a function of its temperature; (3) the lifetime of a laser diode can be decreased significantly in response to a significant rise in temperature; and (4) the power output of a laser diode at a constant drive current is a function of temperature, and will usually increase as the temperature is lowered.
It is therefore desirable to provide an improved heat removal process and device for removing waste heat from laser diodes, which overcomes most if not all of the aforementioned problems.